The invention relates to a guide tube of a nuclear reactor fuel assembly,
Fuel assemblies, and in particular fuel assemblies for water-cooled nuclear reactors, usually comprise a framework in which are inserted and maintained fuel rods constituting a cluster in which the rods are parallel to one another. The framework in particular comprises guide tubes arranged parallel to the rods of the cluster which constitute both structural elements of the framework and elements for guiding absorber rods and control rods used for regulating in service the reactivity of the core of the nuclear reactor constituted by juxtaposed fuel assemblies. The fuel assemblies are disposed in the core of the nuclear reactor in a vertical position, i.e. in such position that the rods of the cluster of the assembly and the guide tubes are vertical. The guide tubes guiding the absorber rods of the control clusters of the nuclear reactor have, in an end part constituting the lower end part of the guide tubes of the assemblies in the service position in the core, a reduced diameter or a narrowing of the section which has for purpose to create a region in which the absorber rods of the control rods of the reactor are braked or slowed down in the case of a dropping of the control rods subsequent to a stoppage of the nuclear reactor which may be a programmed stoppage or a stoppage for an incidental cause. This device for braking the control rods by a throttling of the cooling fluid of the reactor between the absorber rods of the control bar and the lower part of reduced diameter of the guide tubes operates by a braking effect named a dash-pot effect.
The lower part of the guide tubes of the fuel assemblies is highly stressed upon the dropping of the control rods, in particular by compression of the cooling fluid in contact with the internal walls of the guide tubes of the fuel assembly in their region of reduced diameter. In the case of transitional periods of operation of the nuclear reactor, the assembly may be slightly raised by the cooling fluid and the force of inertia of the fuel assembly when it descends may cause a flexion and/or torsion of the part of reduced diameter of the guide tubes which constitutes a weak part of the tube, when the reduction in the diameter of the lower part of the guide tube is obtained by a necking of the tube, the wall thickness being substantially constant throughout the length of the tube.
It has therefore been proposed to employ guide tubes whose lower end part or base is reinforced in such a manner as to withstand the stresses created by the dropping of the control rods and in the course of the transitional periods. A first solution may comprise reinforcing the lower part of the guide tube by a tubular sleeve which is coaxial with the guide tube, placed against and welded to the guide tube at both ends. However, the welds of the reinforced parts of the guide tubes of zirconium alloy which result in residual stresses, may diminish the mechanical resistance and corrosion resistance of the guide tubes. Further, defects in the alignment may occur between the two tube sections to be assembled.
It was therefore proposed in FR-A-2714516 to produce a guide tube with a reinforced base in one piece. The guide tube has a cylindrical outer surface having of substantially constant diameter, except for end regions for fixing the guide tube to the terminal elements of the fuel assembly, and the wall of the tube has a thickness which is increased in a lower part whose length may be between 10 and 30% of the total length of the tube.
The guide tube is obtained by producing a tubular blank whose thickness is substantially constant and this thickness is reduced in a fraction of its length, between 70 and 90%, while maintaining a constant inside diameter. The part of the blank which had maintained its original thickness is then upset toward the interior so as to constitute a tube having a constant outside diameter. The reduction in the thickness and the upsetting may be effected by rotary hammering operations.
This method of obtaining guide tubes of zirconium alloy has been found to be delicate to carry out.
Further, between the part of the tube, termed the main or body part, having a first thickness which was obtained by reduction in the thickness of the blank and the second part of the tube which was upset toward the interior, there is a transition region having an internal surface in the shape of a conical chamfer whose vertex angle has a value of around 10xc2x0. The presence of a discontinuity in the internal cylindrical surface of the guide tube may reduce the quality of the guiding of the absorber rods and limit the dropping speed of the control rods. Further, this transition region may be a weakened region of the guide tube.
For producing tubes sheathing the rods of the fuel assemblies of nuclear reactors of zirconium alloy, a tubular blank is formed in the usual manner by rolling in a pilgrim or pilger rolling mill. The rolling permits obtaining the sheathing tube with its final dimensions in the course of a plurality of successive rolling operations.
This process may also be employed for the shaping of guide tubes of zirconium alloy.
However, such a process has never been employed heretofore for producing a guide tube having a cylindrical outside surface of constant diameter, a body or main part of the guide tube having a first thickness and a lower end part of the guide tube having a second thickness exceeding the thickness of the main part.
An object of the invention is a guide tube for a nuclear reactor fuel assembly having a substantially constant outside diameter and comprising at least one main part which has a first wall thickness and a reinforced part, in a fraction of the length of the guide tube, which has a second wall thickness exceeding said first wall thickness.
The guide is produced by a process which comprises:
effecting, in a first stage, a rolling of a first section of the blank on a first part of the mandrel with a reduction of the outside diameter of the blank to the outside diameter of the guide tube and a reduction of the wall thickness of the blank to one of said first wall thickness and second wall thickness,
displacing the mandrel in the axial direction of the blank, and
effecting, in a second stage, a rolling of a second section of the blank on a second part of the mandrel spaced from said first part in the axial direction of the mandrel, with a reduction of the outside diameter of the blank to the outside diameter of the guide tube and a reduction of the wall thickness of the blank to the other of said first wall thickness and second wall thickness.